1. Field of the Invention
This invention relates to hatch covers for cargo carrying vessels such as the hopper of a railway covered hopper car and latch structures for such hatch covers.
2. Description of the Prior Art
Locking arrangements for lockingly sealing a hatch cover to a hatch opening or a coaming of a vessel such as a lading hopper of a railway covered hopper car are old and well known. The lading in a given vessel may have properties such that excess pressure built up inside the vessel when the hatch is sealed. When the locking arrangement is released the excess pressure may swing the hatch cover open with considerable force endangering human operators and potentially damaging the apparatus. To prevent this, the locking arrangement may be designed to retain the hatch cover in an intermediate "blow-off" position where the hatch cover is free to move enough to vent pressure but restrained against larger movement.
U.S. Pat. Nos. 4,157,146; 4,388,873; and 4,441,431 disclose various arrangements for sealing a hatch cover and retaining it in a blow-off position. U.S. Pat. No. 4,157,146 discloses a plurality of clamps mounted about the hatch circumference or perimeter requiring a plurality of repeated operator functions to open the hatch. U.S. Pat. Nos. 4,388,873 and 4,441,431 each disclose a hatch cover securable by a cam lock in both sealed and blow-off positions but the latch structure is relatively weak and in the event of a mechanical failure the hatch cover might blow open. An additional problem is encountered when human operators use hammers to disengage latch structures of the disclosed configuration by hammering the cam lock to a blow-off position and then hammering it to a disengaged position, risking damage to the latching arrangement.